Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry with a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, and etching). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are external electrical contacts through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. After forming the dies, the wafer is thinned by backgrinding, and then the dies are separated from one another (i.e., singulated) by dicing the wafer. Next, the dies are “packaged” to couple the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines. Conventional processes for packaging dies include electrically coupling the bond-pads on the dies to an array of leads, ball-pads, or other types of electrical terminals, and then encapsulating the dies to protect them from environmental factors (e.g., moisture, particulates, static electricity, and physical impact).
Leaded packages, for example, include a die bonded to a lead frame with the die either seated on a die paddle or attached directly to the leads in a leads-over-chip arrangement. The bond-pads on the die are then wire-bonded to corresponding leads. The lead frame and die may then be encapsulated with a mold compound to form a packaged microelectronic device. In applications in which the leaded package includes a ball grid array, the casing encapsulating the lead frame includes openings at corresponding ball-pads on the leads. The openings are formed by contacting the ball-pads on the leads with corresponding projections in the mold during encapsulation. Next, a plurality of solder balls are placed in corresponding openings and attached to associated ball-pads. After connecting the solder balls, the packaged device can be attached to a printed circuit board or other external device.
One drawback of conventional methods for packaging a leaded device is that the projections in the mold may not contact the ball-pads and/or the mold compound may leak between the projections and the ball-pads. Accordingly, the encapsulated device may include mold flash over part or all of the individual ball-pads. It is difficult to remove the mold flash from the ball-pads without damaging the casing of the device. As a result, some conventional packaged leaded devices have mold flash between a portion of the solder ball and the ball-pad of one or more leads. In these devices, the mold flash may impair the structural and/or electrical connection between the solder ball and the ball-pad and render the devices defective.
Another drawback of conventional methods for packaging a leaded device is that the force of the mold compound flowing into the mold cavity may cause the flexible lead frame to bow or otherwise bend. In certain applications, the lead frame may be molded in a bowed configuration such that several of the solder balls cannot contact the printed circuit board during subsequent attachment, rendering the packaged device defective. Accordingly, there is a need to improve conventional processes for packaging dies attached to lead frames.